Clays and Clay Minerals, Vol. 38, No. 5, 549-555, 1990. REACTION SCHEME FOR THE OXIDATION OF As(III) TO As(V) BY BIRNESSITE JOrINNIE N. MOORE Department of Geology, University of Montana, Missoula, Montana 59812 JEFFREY R. WALKER Department of Geology, Vassar College, Poughkeepsie, New York 12601 THOMAS H. HAYES Department of Chemistry, University of Montana, Missoula, Montana 59812 Abstract--The oxidation of As(III) to As(V) by K-birnessitewas examined at different temperatures, pHs, and birnessite/As(III) ratios. Experiments ranged in duration from 5 to 64 hr, and solution and solid products were determined at several intervals. All experiments showed that the reaction produced large amounts of K+ to solution and very little Mn2+. As(V) was released to solution and incorporated into the K-birnessite. The oxidation was initially rapid and then slowed. The oxidation of As(III) was probably facilitated initially by autocatalytic Mn-As(V) reactions occurring mostly in the interlayer, in which large amounts of As(V) and K+ could be easily released to solution. The reaction also slowed when interlayer Mn was exhausted by forming Mn-As(V) complexes. Mn(IV) could only be acquired from the octahedral sheets of the birnessite. The two-stage reaction process proposed here depended on the layered structure of birnessite, the specific surface, and presence of exchangeable cations in K-birnessite. Key Words--Arsenic, Birnessite, Oxidation, Potassium, Solution. INTRODUCTION The oxidation of As(III) to As(V) by manganese ox- ide minerals is an important means of altering the tox- icity of arsenic in aquatic environments (Oscarson et al., 1980, 1981a, 1981b, 1983). As common particu- lates in fresh and marine environments, various forms of manganese oxides mediate these reactions (Jenne, 1968; Murray, 1975; Martin and Meybeck, 1979; Wan- gersky, 1986; Lind, 1988). Birnessite, a frequently en- countered, natural form of manganese oxide in modern sediments and soils (Jones and Milne, 1956; Giovanoli et al., 1970a), is an important component in the natural oxidation of As(III) to As(V) (Oscarson et al., 1980). Because As(III) is more toxic than As(V) (Coddington, 1986), this reaction controls arsenic toxicity. The structure ofbirnessite is not well known, because the mineral does not form grains large enough for single crystal analysis and is variable in composition. Bir- nessite was originally identified from Mn-rich pans at Birness, Scotland, by Jones and Milne (1956); they equated the mineral to 6-MnO2. Their chemical anal- ysis established an O:Mn ratio of 1.9, and they cal- culated the following formula: (Na0.7Ca0.a)Mna8 +7014" 2.8 H20. (1) Giovanoli et al. (1970a) proposed a probable sheet- like structure for birnessite similar to that ofchalcoph- anite (Giovanoli et al., t970a; Burns and Burns, 1977; Copyright @ 1990, The Clay MineralsSociety Golden et al., 1986; Stouff and Boulegue, 1988). Each sheet consists of edge-sharing MnO6 octahedra. Mn 4+ occupies five of every six octahedral sites; the sixth site is vacant. Between the octahedral sheets, an interlayer may contain Mn 3+, metal cations (M+/M2+), and water molecules. Mn 3+ ions are presumably located above and below the vacant positions, octahedrally coordi- nated by water molecules of the interlayer and oxygen atoms of the MnO6 octahedral sheets. The position of the M+/M 2+ cations is unclear, but they are thought to be positioned between the MnO6 octahedral sheets and the interlayer water sheets. The total layer thickness, d(001), of this arrangement is about 7.2 A. Based on water content determined by dehydration at 600~ the data of Giovanoli et al. (1970b) give the following formula for synthetic Na-birnessite: Na4Mn a'3+14025' 11 H20. (2) This data gives an O:Mn ratio of 1.8. Data based on dehydration at 300~ give an O:Mn ratio of 1.98 (Gio- vanoli et al., 1970b), and would produce a somewhat different formula: Na4Mn 3"7+ 14028' 8 H20. (3) Giovanoli et al. (1970b), however, used neither of these formulae; instead, they presented the following gen- eralized formula: Na4Mn 3"6+ 14027' 9 H20. (4) 549